1. Field of the Invention
This invention relates to method for trapping uncharged multi-pole particles, and, more specifically, for trapping n-pole particles in an (n+4)-pole electric field potential.
2. Discussion of the Related Art
There are many applications in the fields of physics, biology and chemistry that require the separation and detection of molecules of different size, mass, or polarity. In a quadrupole ion trap mass spectrometer, for example, particles (e.g., atoms, molecules) are ionized, trapped inside a quadrupole potential region in a He buffer gas, and subsequently separated according to the ratio of their mass (m) to charge (q), as their orbits become unstable.
Exemplary ion traps are described, for example, by W. Paul et al. in U.S. Pat. No. 2,939,952 issued Jun. 7, 1960. One such ion trap, known as a quadrupole, is described by R. E. March in “Quadrupole Ion Trap Mass Spectrometer,” Encyclopedia of Analytical Chemistry, R. A. Meyers (Ed.), pp. 11848-11872, John Wiley & Sons, Ltd., Chichester (2000). Both of these documents are incorporated herein by reference.
In general, however, ion traps rely on the charged nature of the particles they trap, and, as such, are incapable of trapping uncharged (i.e., electrically neutral) particles.
Thus, a need remains in the mass spectrometer art for an apparatus that is capable of trapping particles that are uncharged.
Uncharged particle detection/separation schemes, unrelated to the quadrupole ion trap technique, are known in two other fields: gas/liquid chromatography and gel electrophoresis. In a gas or liquid chromatograph a solute, combined with a carrier gas or solution, is injected into a temperature controlled column. The components migrate at different speeds depending on the interaction with the stationary phase and are detected separately at the output. On the other hand, in gel electrophoresis nucleic acids and proteins are separated by their diffusion through a gel under an applied external electric field. Recently there have been a number of reports of a micro-fabricated array of sieves on a chip to sort molecules by size and mass with the goal of making a miniaturized bioanalytical system. See, for example, R. H. Austin, et al., IEEE Trans. Nanotech., Vol. 1, No. 12, pp. 12-18 (2002) and T. A. J. Duke et al., Phys. Rev. Lett., Vol. 80, No. 7, pp. 1552-1555 (1998), both of which are incorporated herein by reference. These applications are also expected to benefit from alternative uncharged particle detection schemes of the type described below.